As far as I know, at least in the US, there are only regulations regarding concentrations in the environment for exposure risk assessment. So, for instance, there are regulations regarding concentrations in drinking water, soil, solid waste, and air.
You use hair or other biomonitor medium (blood, etc) more as an indicator that exposure has occurred, and I don't think that there are any set concentrations of mercury exposure that produce a toxic effect as adverse health outcomes related to mercury will vary depending on the species of mercury, the route of exposure, and the weight, age, etc of the person, in addition to the overall concentration.
I hope that this provides some insight into your question, maybe another person will have a different perspective that will be helpful!
I suggest a recent European Food Safety Authority Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food
(http://www.efsa.europa.eu/it/efsajournal/doc/2985.pdf) that also study the quantitative relationship between mercury levels in hair and blood and daily average or intake levels of mercury (especially methy-mercury).
In an our little experience, total mercury levels, being so strictly related to mercury food content, ranged from 0.5 to 1.5 ppm in hair, with the highest levels in frequently fish consumers.